Cylinder dryer with felt



April 7, 1970 R. GUSTAFSSON CYLINDER DRYER WITH FELT 2 Sheets-Sheet 1 Filed March 14, 1968 FIG. I.

INVENTOR; GUSTAFSSON 4 W RUNE ATTYS- April 7, 1970 R. GUSTAFSSON 3,504,443

CYLINDER DRYER WITH FELT Filed March 14, 1968 2 Sheets-Sheet 2 mvcmon: BY RUNE GUSTAFSSON ATTYS.

United States Patent 3,504,443 CYLINDER DRYER WITH FELT Rune Gustafsson, Stockholm, Sweden, assignor to Aktiebolaget Sveuska Flaktfabriken, Stockholm, Sweden Filed Mar. 14, 1968, Ser. No. 713,060 Claims priority, application Sweden, Mar. 28, 1967, 4,181/ 67 Int. Cl. F26b 11/02 U.S. Cl. 34117 6 Claims ABSTRACT OF THE DISCLOSURE A cylinder dryer in which the material to be dried is pressed between the inner surface of an endless, highlypermeable dryer felt and the outer surface of a heated rotating cylinder. Moisture-laden drying medium is mechanically removed from the outer surface of the dryer felt by a deflecting body which is made of resilient material and projects towards the surface of the felt while it is on the cylinder. Moisture removal may be facilitated by blowing drying medium against the felt downstream of the body and withdrawing drying medium from the surface of the felt upstream of the body.

The present invention relates to an apparatus for increasing the drying intensity in a cylinder dryer in which a material to be dried is pressed by an endless highly permeable dryer felt against the surface of a heated rotating cylinder.

A great number of the-preferably web-shapedmaterials subjected to drying on an industrial scale are dried by being pressed against the surface of rotating cylinders which inwardly are heated by steam. This method is wellestablished in the drying of, for example, textile, paper pulp, paper and cardboard. When the material to be dried is web-shaped and of a strength so as to be able to take up appreciable tensile stresses, it may be pressed against the cylinder surface by applying a tensile stress in the longitudinal direction of the web. The material not being web-shaped or having suflicient tensile strength, the pressing is effected by feeding the material to be dried between the cylinder surface and an endless mat, a so-called dryer felt, which follows along and abuts to the cylinder surface. A felt of this type is in most cases a strong textile fabric, the thickness of which must be adjusted to the forces which the felt is desired to take up without breaking. The thickness, thus, may be relatively great. The felt obstructs to a high degree water vapour to leave the drying goods and thereby extends the drying time.

The introduction and progress of the synthetic fibers latterly have rendered it possible to replace the strong fabric of the drying felts made of natural fibers by a fabric of synthetic fibers. Thanks to the greater strength of synthetic fibers, the fabric can be made considerably looser than a fabric of natural fibers, which increases its permeability by many times and thereby increases the drying capacity of the arrangement. Dryer felts having high permeability hereinafter will be called highlypermeable dryer felts.

Upon a relative motion between a turbulent fluid and a solid body, close to the surface of the body a border layer of fluid is formed which has a lower speed relative to the body than the remaining part of the fluid. At heat transfer between a fluid of turbulent flow and a solid body, the heat transfer resistance is supposed to be entirely within the border layer. Consequently, for obtaining a great heat and material exchange between the fluid and the body, the border layer must be given a small thickness.

The inventor now has found that the border layer adjacent the surface of a running highly permeable dryer felt grows very rapidly and, therefore, reaches a substantial thickness in a short time. The border layer thereby exercizes an unnecessarily high resistance to the evaporation from the material to be dried. The water vapour leaving the drying material is braked by the border layer which thereby rapidly is concentrated with water vapour. The evaporation also per se contributes to an increase of the border layer thickness.

It further was found by the inventor, that the border layer simply and effectively can be removed in a purely mechanical way. The invention, therefore, is characterized in that the border layer of moist drying medium which owing to the dryer felt motion in relation to the surrounding medium is formed adjacent the surface of the dryer felt on that side facing away from the cylinder, and which border layer in relation to the dryer felt partly stands still and partly moves slowly, at least in one point along the contact of the felt with the goods to be dried is mechanically removed from the felt surface. Hereby the water vapour can leave the drying goods more easily whereby the evaporation intensity and the drying capacity increase.

An apparatus for carrying out the method according to the invention comprises a cylinder dryer at which the material to be dried is pressed by an endless highly permeable dryer felt against the surface of a heated rotating cylinder, and is characterized in that a body of resilient material is mounted adjacent the felt surface on its side facing away from the cylinder along at least one line substantially in parallel with the cylinder axis. By attaching purely mechanical means for the removal of the border layer, the apparatus can be given a compact and stable design and be driven with low effect consumption.

One embodiment of the apparatus is characterized in that the resilient body is shaped like a brush sliding with its brushes on the felt surface. Due to the penetration of the brush also in between the meshes of the dryer felt fabric, the border layer is removed very effectively.

Another embodiment of the apparatus is characterized in that the resilient body is shaped like a doctor blade having its edge directed against the direction of the felt motion and slightly spaced from the felt surface. Thanks to this embodiment neither the resilient body nor the dryer felt are subjected to any wear. The border layer, however, is not removed as effectively as in the foregoing embodiment.

The invention also is unconventional in that respect that a flow border layer usually is not removed by purely mechanical means. Conventionally the border layer thickness is held small by means of a high parallel relative speed between the fluid and the surface, or by blowing perpendicularly against the surface. In the present case, however, a purely mechanical removal has been found to render a substantial effect, which can be increased by blowing air against the running felt subsequent to its passage of the resilient body. An embodiment of the apparatus, therefore, is characterized in that the resilient body is mounted on a hollow holder extending transverse to the direction of felt motion, which holder close to the resilient body and on its side facing with the direction of felt motion is provided with openings communicating with the hollow, and that said hollow also communicates with the pressure side of a transport means for gases.

The effect can be increased still more by a conrolled removal by suction of the air mechanically removed from the border layer, which air is highly concentrated with water vapour. A last embodiment of the apparatus, therefore, is characterized in that the resilient body is attached on an elongated hollow holder extending transverse to the direction of felt motion, the hollow of the holder being divided by a longitudinal partition wall into two chambers which via openings located adjacent the resilient body communicate with the environment of the holder, in such a manner, that all openings of one of said chambers are located in that of the holder sides which seen in the direction of felt motion is located before the resilient body, and that said chamber also communicates with the suction side of a transport means for gases, all openings of the other of said chambers being located in that of the holder sides which seen in the direction of felt motion is located behind the resilient body, and said chamber also communicating with the pressure side of a transport means for gases.

The invention will be described in greater detail in the following, with reference to the accompanying drawings on which 'FIG. 1 shows a simple embodiment of the invention,

FIG. 2 shows a drying section with the apparatus according to FIG. I placed therein,

FIG. 3 shows an embodiment comprising blowing against the felt, and

FIG. 4 shows an embodiment comprising both sucking away and blowing against the felt.

Referring to FIG. 1, a resilient body 1 in the form of a rubber strip slides against the surface of a highly permeable dryer felt 2, which presses a paper Web 3 against the surface of a hollow steam-heated cast-iron cylinder 4. The object of the felt 2 is to establish intimate contact between the heated cylinder surface and the paper web 3 in order thereby to cause the paper web to assume a temperature which as far as possible is equal to that of the cylinder surface. Water evaporates from the paper web and diffuses through the felt 2 out into the ambient. Immediately above the surface of felt 2 a so-called border layer of air is formed which in relation to the felt stands still or moves with a low speed. The transport of the evaporated water vapour through the border layer to a great extent must be effected by diffusion and, therefore, proceeds slowly. The border layer, thus, constitutes an obstacle to the evaporation.

The border layer, however, can be prevented from remaining at the surface of the felt 2, by letting a strip 1 of rubber slide against the felt 2. The border layer is broken by the strip 1, and behind the strip 1 immediately a new border layer will be formed which, however, first after a certain period reaches such a thickness as substantially to impede evaporation. By placing a strip like the strip 1 spaced from strip 1 in a relationship suitably adjusted to the web speed, the border layer thickness can be held within desired limits. The strip 1 extends across the entire width of felt 2 and is secured at a carrying beam 6.

FIG. 2 shows the apparatus according to FIG. 1 disposed at several cylinders in a cylinder drying part for paper. The dryer felts 2 are deflected between the drying cylinders 4 over guide rollers 5. The paper web 3 runs up and down between the cylinders 4 and is pressed against the cylinders by the felts 2. At each of the cylinders 4, half-way between the first and the last contact point of the felt with every cylinder, a resilient body 1 is provided according to FIG. 1 in sliding relationship on the felts 2. The felts, when running inwards against the cylinder surfaces, are already then surrounded by air standing still in relation to the felts. Upon the contact with the paper web this air is pressed over to that side of the felts facing away from the paper and forms a border layer which is concentrated with water vapour as the felts run about the cylinders. At the rubber strips 1 the border layer is scraped off the felt surfaces. Immediately behind the strips 1 new border layers commence to develop. In the embodiment shown the felts run at such a high speed that the new border layers are not given sufilcient time for growing to an appreciable thickness, before the felts get out of contact with the paper web. It is, therefore, sufiicient to provide one resilient body 1 for every cylinder.

FIG. 3 shows a different embodiment of the resilient body. The body is shaped as a doctor 7, the edge of which is directed against the running direction of the felt and like a knife cuts off the greater part of the border layer at the surface of the felt 2. The doctor 7 does not touch the felt. Thus, neither the doctor nor the felt are subjected to wear. The gap which must be found between the doctor edge and the felt allows a certain part of the border layer to remain on the felt when it is passing the doctor. This is of minor importance, for two reasons. Firstly, the remaining part has a small thickness in relation to that of the part scrapped off, so that the effect achieved, nevertheless, is substantial. Secondly, the felt and the rear side of the doctor form a diverging passageway in which the speed of the border layer is reduced with increasing pressure. This in its turn involves, that the static pressure in the smallest section between the doctor edge and the felt is lower than in the ambient, which results in an acceleration of the border layer. Thus, the speed of the border layer in relation to the felt varies, and the border layer is losing its character. Finally, the air advancing in the smallest section between the doctor edge and the felt, partly is attracted by the rear side of the doctor, due to friction phenomenons, and thereby brings about a further thinning of the border layer. For being able to utilize the said phenomenons, the angle between the rear side of the doctor and the Wire must not be too great. The doctor 7 which is made of plastics, is soft and resilient in order not to damage the felt when a foreign object is pressed out from the cylinder surface.

In FIG. 3 further is shown an eifect increasing blowing against the felt behind the resilient body 7. For this reason, the beam carrying the resilient body is shaped as a tube 8 provided with a plurality of openings 9 located to the side of each other in the longitudinal direction of the tube 8. The tube is supplied with air under pressure, whereby the air through the openings 9 is blown against the felt 2 in the form of jets which together cover the entire width of felt 2. The jets can penetrate into the fabric of the felt 2 and remove the water vapour possibly collected there. At the same time, the last remainders of the border layer are removed.

FIG. 4 shows a further embodiment of the resilient body, which is designed as a brush 10 with its brushes sliding against the felt surface. The brush 10 treats the felt 2 more gently than strip 1 in FIG. 1 and renders a somewhat better effect.

FIG. 4 also shows a different embodiment of the carrying beam. The beam is a tube 11 which by a longitudinal partition wall 12 is divided into two chambers 13 and 14. Both chambers are provided in the surface of tube 11 with openings 15 and 16 respectively. The chamber 14 communicates with the suction side of a fan. Through the openings 16 which are located in a row along the length of the tube before the brush 10, moist air is sucked in from the border layer removed by the brush 10, so that the moist air is not spread in the drying section. The other chamber 13 communicates with the pressure side of a second fan, in such a manner, that through its openings 15 dry air is blown against the felt 2 behind the brush 10. By sucking off water vapour in this way from the place where it is most concentrated, the partial pressure of the water vapour in the drying section can be held low without applying strong forced ventilation, which would involve energy consumption and heat losses. The drying sections of the type here referred to usually are screened off from the environment by a casing within which the partial pressure of the water vapour is held at the desired value by forced ventilation.

The embodiments shown of the resilient body are not bound to the embodiment of the carrying beam, with which they are assembled in the drawings, but they can freely be combined with any type of beam.

What is claimed is:

1. In a cylinder dryer in which material to be dried is pressed by the inner surface of an endless, highly-permeable dryer felt against the outer surface of a heated r0- tating cylinder, a body of resilient material mounted adjacent the outer surface of the felt and having an edge disposed transversely of the dryer felt and directed toward the felt along a line substantially parallel to the cylinder axis and intermediate the first and last contact points of the felt with the cylinder so as to mechanically remove moist dry medium from the border layer formed adjacent the exposed surface of the dryer felt when the felt passes its first contact point with the cylinder.

2. A cylinder dryer according to claim 1 including a plurality of cylinders and a resilient body for each cylinder, wherein said line is substantially half-way between the first and last contact points of the felt with each cylinder.

3. An apparatus according to claim 1 characterized in that the resilient body is shaped like a brush with its brushes sliding against the surface of felt.

4. An apparatus according to claim 1 characterized in that the resilient body is shaped like a doctor having its edge directed against the direction of motion of the felt and slightly spaced from the surface of felt.

5. An apparatus according to claim 1 including a transport means for gases characterized in that the resilient body is mounted on a hollow holder extending transverse to the direction of felt motion, which holder close to the resilient body and on its side facing with the direction of felt motion is provided with openings communicating with the hollow, and that said hollow also communicates with the pressure side of said transport means to blow away additional moist drying medium from the border layer.

tion between each chamber and the exterior of said holder,

all of the openings of one chamber being located in advance of resilient body, all of the openings of the other chamber being located behind the resilient body, means for applying suction to one chamber to withdraw medium in advance of said body from said felt during its travel,

and vmeans to blow medium into other chamber to thereby blow medium behind the resilient body against said felt in its travel.

References Cited UNITED STATES PATENTS 3,314,162 4/1967 Haywood 34-111 3,316,657 5/1967 Haywood 34111 3,337,968 8/1967 Krikorian et a1. 34-111 3,406,464 10/1968 Clark 34-111 JOHN J. CAMBY, Primary Examiner US. Cl. X.R. 34-123 

